1. Field of the Invention
This invention relates to a device and a method for generating a reproducible, uniform and ultra-pure molecular beam of elemental molecules, utilizing a refractory effusion cell. In particular, the invention relates to a filament construction able to produce a substantially constant temperature along the long axis of the crucible of an effusion cell and provide long-term flux stability.
2. Description of Related Art
Molecular beam epitaxy (MBE) is a crystal growth technique intended to allow precise control of beam fluxes and deposition conditions. For successful MBE growth, it is important to produce a stable, reproducible and uniform molecular beam of each elemental constituent of the thin film being deposited. This is accomplished, to some degree, through the use of Knudsen or effusion cells. Ideally, a Knudsen cell is an isothermal enclosure with an infinitesimally small exit aperture bounded by vanishingly thin walls. Knudsen-like cells are critical components in MBE systems and are the basis for substantial molecular beam generation.
Typically, MBE sources differ significantly from ideal Knudsen cells. Exit apertures are not bounded by vanishingly thin walls and large exit orifices are often employed to enhance growth rates. In conventional cell design, the solid or liquid source materials are held in an inert crucible heated by radiation from a resistive heat source. A thermocouple is used to provide temperature feedback. Conventional cells have significant radiated heat loss at the orifice, resulting in a temperature drop at or near the orifice and large thermal gradients along the long axis of the crucible. In certain situations, source material may condense at the opening of the crucible, reduce the exit orifice and redistribute the contents of the crucible. This causes a dramatic change in flux density for a given temperature set point. To maintain the same flux density over an extended period of time, a continuous increase in applied power to the cell must be maintained. Unfortunately, this may result in an increase in the generation of active gases such as nitrogen and carbon monoxide. A reduction in the cell's exit orifice dimensions may also decrease the uniformity of film growth.
Conventional cell design can allow certain source materials, such as gallium, to condense at the orifice and form droplets which roll back into the heated source material producing microdroplets of source material spitting from the thermodynamic reaction. Such spitting may land on the substrate and cause a defect.